Race condition in `VisitQueue::pop`

[Boscop]

If 2 (or more) threads call pop at the same time and last > 0, they both find that last > 0 and both decrement last, then last underflows, (and then it does unsafe { self.data.get_unchecked(last - 1) } with that value):

HVM/src/runtime/data/visit_queue.rs

Lines 54 to 58 in 9a6f6ff

 let last = self.last.load(Ordering::Relaxed); 
 if last > 0 { 
   self.last.fetch_sub(1, Ordering::Relaxed); 
   self.init.fetch_min(last - 1, Ordering::Relaxed); 
   let visit = unsafe { self.data.get_unchecked(last - 1) }.swap(0, Ordering::Relaxed);